Postal services are held accountable for achieving certain service levels of performance, and one particularly important criterion is on-time delivery. Postal services typically measure their on-time delivery performance by assessing the effectiveness of both the sorting system, and the delivery operations. In many countries, the target is 97% to 98% of first class mail delivered within one day of receipt by the postal service (hereinafter “the post”). Typically, the targets for standard class mail are less challenging: for example, 95% of mail delivered within three to five days of receipt by the post.
In centralized postal sorting systems, mail is usually passed through automated sorting systems multiple times. The United States Postal Service (USPS), for example, has invested in sufficient sorting equipment to be able to sort 80% of letter mail to delivery sequence before it leaves a centralized sorting facility. To accomplish this level of sorting, often the mail will be passed through the sorters between four and six times. Because the delivery commitments for first class mail are so demanding, the first class mail is generally sorted as soon as it is available. In typical centralized sorting operations, once the first class mail has been sorted, the operators will sort as much standard class mail as they can within the time periods available for sorting.
After the last pass through the sorters, the mail must be placed in mail trays and loaded onto trucks by the deadline for dispatching the mail to the delivery offices. Typically this deadline for dispatch is between 4:00 and 6:30 A.M., depending upon the distance of the delivery offices from the centralized sorting facility. If the mail is late being dispatched from the centralized sorting facility, it often arrives much later at the delivery offices, due to delays caused by rush hour traffic. So, the posts tend to be fairly rigid in insuring that the mail is on the truck and on the way to the delivery offices no later than the established dispatch deadlines.
The problem is determining how much standard mail the operators should mix in with the first class mail during the multiple sorting operations. Sometimes, this mixing of standard mail with first class mail is limited to the last two passes through the sorting machines. And, because the performance of the sorter is somewhat affected by the type of mail being fed, and the skill of the operators, the ability to predict the total time to complete each pass through the sorters is an approximation based on experience of the operators and supervisors. Supervisors will occasionally get that approximation wrong, due to variables that they cannot control, and they consequently miss the dispatch deadline, or will need to dispatch some portion of the mail before it is completely sorted. These uncontrollable variables include the skill and efficiency of the operator, the number of jams and other shutdowns of the sorting equipment, and variables in the mail itself, such as the thickness and size of the mail pieces. In order to minimize the possibility of missing the dispatch deadline, some supervisors will err on the side of caution, and instruct the sorter operators to hold back some of the standard class on the second to last run in order to make sure that the last run through the sorter can be completed prior to the dispatch deadlines.
Therefore, sorting operations are often not as efficient as they could be. The total volume of mail run through the sorters falls well short of the ideal. And, mail that is run through the sorters—but that does not finish the last pass or two through the automated sorting equipment—is sometimes dispatched unsorted to the delivery offices, where it is sorted by hand. Manual sorting of mail is the most time-consuming and expensive way to process mail.
What is needed is a way to know precisely how much mail is to be sorted in the last pass or two, and precisely how long it will take to sort that mail so that the maximum amount can be sorted automatically, while still ensuring that the dispatch deadlines are met. This problems exist both for conventional sorters, as well as for damp-based sorters wherein mail is put in clamps, and the mail is sorted by manipulating the clamps instead of by directly guiding the mail pieces. Examples of such a clamp-based system can be found in International Application WO 2006/063204 filed 7 Dec. 2005 titled “System and Method for Full Escort Mixed Mail Sorter Using Clamps” and can also be found in U.S. Provisional Application No. 11/519,630 filed 12 Sep. 2006 titled “Sorter, Method, and Software Product for a Two-Step and One-Pass Sorting Algorithm,” which are both incorporated herein by reference in their entirety. The problem also exists for macro-sorters, which are sorters that simultaneously sort inbound as well as outbound mail. The concepts of macro-sorting are described, for example, in U.S. Provisional Application No. 60/669,340 filed 5 Apr. 2005, titled “Macro Sorting System and Method” which also incorporated herein by reference in its entirety.